Nonequilibrium boundary layer at a stagnation point for a hydrogen-helium stream over ablating graphite

Abstract

The nonequilibrium axisymmetric stagnation point boundary layer over an ablating graphite surface is considered. The external stream is a high temperature mixture of hydrogen and helium. Variable thermodynamic and transport properties are used. Leonard-Jones potential model is used to calculate the transport coefficients of each species. Although the mixture rules for viscosity of the gas mixture are used, the weighting functions are more sophisticated than have commonly been used. For the conductivity of the mixture, generalized Wassiljewa coefficients are used. Seven species with 28 dissociation/recombination reactions are considered. Hansen's model for the dissociation rate constants is employed. The recombination rate constants are obtained by invoking detailed balance principles assisted by the JANAF thermodynamic data and the Hansen-Pearson thermodynamic data for C 3. The Knudsen-Langmuir equations with specified vaporization coefficients are incorporated in the surface boundary conditions to investigate nonequilibrium vaporization processes. The equations are used to obtain the heterogeneous vaporization rates. These are combined with convective and diffusive terms at the wall to satisfy species conservation. This gives rise to mixed type boundary conditions for the species. Numerical instability due to finite rate chemistry and massive blowing was encountered, and the methods to overcome the difficulties are discussed. The differencing scheme employed results in a matrix of block-tridiagonal form and a direct factorization method is used to solve the matrix. Solutions corresponding to flow conditions of Jovian and Saturnian atmospheric entry are obtained. Based on currently reported vaporization coefficients, it is found that the correct chemistry assumption for graphite planetary probes is frozen gas phase and finite rate vaporization.